Optical encoders, for example angular, are used like potentiometers, for example for the manual control of electronic equipments which are sensitive to an input parameter that can vary continuously or almost continuously, but they are much more reliable than potentiometers. Typically, in an application in respect of aeronautical equipment, it is possible to use an optical angular encoder to indicate to an automatic piloting computer an altitude or speed setpoint that the pilot chooses by actuating a control button which causes the encoder to revolve. The reliability of the encoder and of the information that it delivers is then an essential element of the encoder.
An optical angular encoder typically includes a disc bearing regular marks, this disc being rotated using a (for example manual) control button. A photoelectric cell fixed in front of the disc detects the marks past of the successive marks when the control button causes the disc to revolve. The marks are typically apertures in an opaque disc, a light-emitting diode being placed on one side of the disc and the photoelectric cell being placed on the other side.
Each passby of a mark constitutes an increment of one unit in the counting of the rotation of the disc. The angular resolution is determined by the angular interval of the marks arranged regularly over a revolution of the disc. To detect at one and the same time increments and decrements of angle of rotation when the direction of rotation is reversed, two photoelectric cells are provided, mutually offset physically by an odd number of quarter intervals. Thus, the lit/unlit logic states of the two cells are coded on two bits which successively take the following four values: 00, 01, 11, 10 when the disc rotates in one direction and the following four successive values 00, 10, 11, 01 when the disc rotates in the other, so that it is easy to determine not only the occurrence of an increment of rotation (change of state of one of the bits) but also the direction of rotation (by comparison between a state of the cells and the immediately earlier state).
The light-emitting diodes are mounted on a fixed printed circuit. The photoelectric cells are mounted on another fixed printed circuit. A revolving disc comprising windows passes between the two printed circuits. A fixed diaphragm, interposed between the revolving disc and the printed circuit carrying the light-emitting diodes, makes it possible to improve the precision of the encoding.
This solution requires two printed circuits, wiring between the circuits and precise relative positionings of the various constituents of the device. All this returns the encoder complex and industrially difficult to produce. Moreover, the assemblage of electronic elements associated with the wiring and with the micromechanics returns this type of encoder architecture fragile.